For a given power input light output from a light source can vary between that from a relatively small size source (say a light emitting diode) to that from a relatively large size source (say a resistive filament). A relatively high powered source tends to generate light more efficiently than, a low powered one. However high intensity light in a display device can dazzle a viewer.
One way to overcome the matter of dazzle is to configure the illumination system so that direct viewing of the illuminating means cannot occur. Another way is to overcome dazzle is to provide an optical filter but this necessarily attenuates the light output so rendering superfluous the use of a relatively high powered source.
The matter of dazzle is not usually a critical matter in connection with an illuminating means where the viewer is meant to look at an object illuminated by the illuminating means rather than the means itself. However a display device is intended to be viewed directly and consequently any output display by means of the device should not contain a viewable region of dazzling brightness.